Three-dimensional printing system with laser calibration system

ABSTRACT

A three-dimensional printing system is configured to selectively solidify a build material at a build plane in a layer-by-layer manner. The three-dimensional printing system includes a laser module, a scan module, and a controller. The laser module is for emitting a light beam along a main optical path from the laser module to the build plane. The scan module includes a motorized mirror and a sensor. The motorized mirror includes a substrate having an optical coating that reflects at least 90% of incoming beam power such that the mirror transmits no more than 10% of the incoming beam power. The sensor is positioned to receive transmitted light from the mirror. The controller is configured to operate the laser module to emit the light beam along the main optical path, analyze a signal from the sensor, and based upon the analysis, to estimate a calibration error for the laser module.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application claims priority to U.S.Provisional Application Ser. No. 62/691,096, Entitled “THREE-DIMENSIONALPRINTING SYSTEM WITH LASER CALIBRATION SYSTEM” by Guthrie Cooper, filedon Jun. 28, 2018, incorporated herein by reference under the benefit ofU.S.C. 119(e).

FIELD OF THE INVENTION

The present disclosure concerns an apparatus and method for the digitalfabrication of three-dimensional articles of manufacture by alayer-by-layer solidification of a build material. More particularly,the present disclosure concerns an advantageous method of calibrating alaser module within a three-dimensional printing system.

BACKGROUND

Three-dimensional printers are in wide use. A number of differentthree-dimensional printing systems utilize lasers for a layer-by-layersolidification of a build material into a three-dimensional article.Challenges with lasers include a need for alignment and calibration.This is particularly true when a new laser is installed. Such aninstallation typically requires the skills of a highly trainedtechnician. Also, laser-based systems tend to experience a “drift” ofalignment and calibration over time with a resultant loss of quality inmanufacturing three-dimensional article. There is a need for a practicalsolution to reduce a cost of maintenance to make high qualitymanufacturing practical.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram schematic of an embodiment of athree-dimensional printing system incorporating various calibrationtechniques.

FIG. 2 is an isometric illustration of an embodiment of a light engine.

FIG. 3 is an electrical block diagram illustrating a scanning system forscanning a light beam over a build plane.

FIG. 4 is a schematic diagram illustrating a portion of a scanningsystem including an X-mirror and an X-sensor.

FIG. 5 is a schematic diagram illustrating a portion of a scanningsystem including a Y-mirror and a Y-sensor.

FIG. 6 is a schematic block diagram of a portion of a three-dimensionalprinting system.

FIG. 7 is a flowchart depicting an embodiment of a method for aligning alight beam to a main optical path.

FIG. 8 is a flowchart depicting an embodiment of a method for analyzinga focus of a light beam.

FIG. 9 is a flowchart depicting an embodiment of a method for aligningthe scanning of a light beam to a build plane.

FIG. 10 is a flowchart depicting an embodiment of a method of operatingthe three-dimensional printing system.

SUMMARY

In a first aspect of the disclosure, a three-dimensional printing systemis configured to selectively solidify a build material at a build planein a layer-by-layer manner. The three-dimensional printing systemincludes a laser module, a scan module, and a controller. The lasermodule is for emitting a light beam along a main optical path from thelaser module to the build plane. The scan module includes a motorizedmirror and a sensor. The motorized mirror includes a substrate having anoptical coating that reflects at least 90% of incoming beam power suchthat the mirror transmits no more than 10% of the incoming beam power.The sensor is positioned to receive transmitted light from the motorizedmirror. The controller is configured to operate the laser module to emitthe light beam along the main optical path, analyze a signal from thesensor, and based upon the analysis, to estimate a calibration error forthe laser module.

In one implementation the laser module is coupled to an aligner that isconfigured to adjust a position of the laser. The adjustment can beconfigured to adjust one or more of pitch angle, yaw angle, and a linearposition along the direction of beam propagation from the laser module.The pitch angle and yaw angle are measured along axes that areperpendicular to the direction of beam propagation from the lasermodule.

In another implementation, the motorized mirror transmits between 0.1and 4.0 percent of the incoming light beam power. More particularly, themotorized mirror transmits between 0.2 and 2.0 percent of the light beampower. Yet more particularly, the motorized mirror transmits about 1.0percent of the light beam power.

In yet another implementation, the motorized mirror substrate includes afront side and a back side. The front side includes the optical coating.The back side includes an opaque feature. The opaque feature can includeone feature or a plurality of features. The opaque feature can be a dotplaced proximate to a central location on the mirror. The opaque featurecan be a reflective material such as a metal thin film. The transmittedlight reaching the sensor includes a shadow formed by the opaquefeature. The controller analyzes the transmitted light and compares acentroid of the shadow relative to a centroid of the beam to determinealignment of the beam to the mirror.

In a further implementation, the motorized mirror includes two mirrorsincluding an X-mirror and a Y-mirror. The mirrors are configured to scanthe light beam over the build plane. The mirrors can have associatedsensors including an X-sensor and a Y-sensor. The controller can analyzetransmitted light from the sensors and determine two differentcalibration parameters from the two sensors. The two differentcalibration parameters can include two or more of a laser focusparameter, a yaw alignment parameter for the laser module, a pitchalignment parameter for the laser module, and a laser power parameter.

In a yet further implementation, the controller is configured to analyzean intensity distribution of the transmitted light. The controller canthen determine whether the laser module is properly focused on the buildplane based upon the analysis.

In another implementation, the light beam converges at least between themirror and the build plane along the main optical path. The light beamalso converges along a secondary optical path between the mirror and thesensor. With an equivalent optical path length, the light beam diameterat the sensor can be the same as the light beam diameter at the buildplane. A converging optical element such as a convex lens can be placedbetween the mirror and the sensor in order to shorten a length of thesecondary optical path.

In yet another implementation, the controller is configured to send analert to a user interface when the calibration error exceeds athreshold. The alert can include instructions for manually correctingthe calibration error. The manual correction can include operating amanual aligner to correct one or more of a laser module focus, a lasermodule pitch, a laser module yaw, and other alignment parameters.

In a further implementation, the laser module includes an adjustmentmechanism. The controller is configured to automatically operate theadjustment mechanism to correct the calibration error when thecalibration error exceeds a predetermined threshold. The adjustment caninclude correction of one or more of a laser module focus, a lasermodule pitch, a laser module yaw, and other alignment parameters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram schematic of an embodiment of athree-dimensional printing system 2 incorporating various calibrationtechniques. Three-dimensional printing system includes a light engine 4,a resin vessel 6, and a user interface 8 all coupled to a controller 10.

The resin vessel 6 contains photocurable resin 12 for forming athree-dimensional article 14 upon a top surface 16 of a build platform18. The build platform 16 is vertically positioned in the resin 12 by avertical positioning mechanism 20.

The light engine 4 includes a laser module 22 coupled to an aligner 24.Aligner 24 provides mechanical support, positioning, and alignment forlaser module 22. The laser module 22 emits a light beam 26 that travelsalong a main optical path 28. The main optical path 28 is defined bylaser module 22 and scan module 30. The main optical path 28 ends at abuild plane 32 which is proximate to a top surface of thethree-dimensional article 14.

The scan module 30 scans the light beam 26 along two substantiallyperpendicular lateral directions (X and Y) to define the build plane 32.The scan directions are substantially perpendicular to a propagationdirection S of the light beam 26. The scan module is mounted to a basehousing 33. The base housing 33 defines an opening 34 (FIG. 2) thatallows the light beam to pass out of the light engine 4 and to the resinvessel 6. Along the main optical path 28 between the laser module 22 andthe build plane 32 the light beam 28 converges from an initial diameterto a final diameter at the build plane 32. In one embodiment, the buildplane 32 is at a point of focus for the light beam 28. However, thelaser module 22 can include optics to controllably vary a diameter ofthe light beam 28 at the build plane 32.

The three-dimensional article 14 is manufactured with a layer by layerprocess under control of controller 10: (1) The vertical positioningmechanism 20 is operated to position an active surface 17 (initially thetop surface 16 of the build platform 18) at build plane 32. (2) A layerof uncured resin 12 is provided over the active surface 17. (3) Thelaser module 22 and scan module 30 are operated to selectively cure alayer of the three-dimensional article 14 onto the active surface 17.(4) Steps (1)-(3) are repeated to complete manufacture of thethree-dimensional article 14. Thus, the material layers of cured resinare accreted onto the active surface 17 in a layer-by-layer manner.

The user interface 8 can be used by the user to input commands and toreceive information from the controller 10. In one embodiment the userinterface 8 is integrated into a chassis of the three-dimensionalprinting system 2. In other embodiments, the user interface 8 is part ofa physically separated computer that can include one or more of asmartphone, a laptop computer, a desktop computer, a tablet computer, orany other device. The user interface 8 can communicate with thecontroller 10 using a wireless or wired connection.

The commands input into the user interface 8 can be for calibration ormanufacture of the three-dimensional article 14. The information caninclude a status, error, or calibration information and can occur duringformation of a three-dimensional article 14 or during a calibrationroutine. The information can be displayed on the user interface 8 and/orstored in a form that can be used later such as in records of a databaseor spreadsheet.

In an alternative embodiment, the resin vessel includes a transparentsheet defining a lower surface. The build platform supports a lowerfacing surface of the three-dimensional article in facing relation withthe transparent sheet. The build plane is defined proximate to a lowerface of the three dimensional article. The light engine is configured toemit light upwardly through the transparent sheet and to the build planeto accretively form layers onto the lower face. The light engine caninclude a light source and a spatial light modulator.

FIG. 2 is an isometric illustration of an embodiment of the light engine4. The laser module 22 is coupled to the scan module 30 with precisionrods 36. Between the laser module 22 and the scan module 30 is a smalldiameter concave lens 38 and a large diameter convex lens 40.

The scan module 30 includes a motorized X-mirror 42 and a motorizedY-mirror 44. The X-mirror scans the light beam 26 along an X-axis andthe Y-mirror scans the light beam 26 along a Y-axis to address the buildplane 32. The main optical path 28 extends from the laser module 22,through lenses 38 and 40, to the X-mirror 42, to the Y-mirror 44,through the opening 34, and to the build plane 32 (FIG. 1). Between thelaser module 22 and the build plane 32, the light beam 26 convergesalong the optical path 28. Stated another way, a diameter of the lightbeam 26 decreases along the optical path 28 toward the build plane 32.In one embodiment, the light beam has a focal point and minimum beamdiameter at the build plane 32. However, the controller 10 can move thelens 38 to control a diameter and degree of focus of the beam at thebuild plane 32.

For purposes of calibration there are at least two secondary opticalpaths. The main optical path 28 reflects from X-mirror 42 and fromY-mirror 44. A secondary optical path 46 extends through the X-mirror 42to an X-sensor 48. Another secondary optical path 50 extends through theY-mirror and to a Y-sensor 52. The secondary optical paths 46 and 50will be described in more detail infra.

The scan module 30 is mounted to base housing 33. Also mounted to thebase housing 33 is a cantilevered support 35 that extends from aproximal end 37 to a distal end 39. The scan module 30 is mounted at theproximal end 37. The secondary optical path 50 is defined along thecantilevered support between the Y-mirror 44 and the Y-sensor 52. Asensor 41 is mounted at the proximal end 37 to receive internal(internal meaning originating from within the printing system 2)vibrations generated by the scan module 30. The sensor 41 provides asignal to the controller 10 that is indicative of the internalvibrations. Sensor 41 can be a multi-axis accelerometer. A sensor 43 ismounted at the distal end 39 so as to be sensitive to vibrationsoriginating from sources that are external to the three-dimensionalprinting system 2. The sensor 43 provides a signal to the controller 10that is indicative of the vibrations at sensor 43. Sensor 43 can be amulti-axis accelerometer. In one implementation, the controller 10 canutilize a signal from sensor 52 to analyze vibrations from externalsources and so there may be no need for sensor 43. In anotherimplementation, the controller 10 utilizes signals from both sensors 43and 52 to analyze the external vibrations.

FIG. 3 is an electrical block diagram including the scanning system 30illustrating the controller 10 coupled to the motorized X-mirror 42,X-sensor 48, the motorized Y-mirror 44, and the Y-sensor 52. Alsoillustrated is the primary optical path 28 and the secondary opticalpaths 46 and 50. Sensor 41 that senses internal vibrations is showncoupled to controller 10. Sensor 43 that senses external vibrations isalso shown coupled to controller 10.

FIG. 4 is a schematic diagram illustrating a portion of the scan module30 including the X-mirror 42 and the X-sensor 48. The X-mirror has asubstrate 54 having a front side 56 and a back side 58. The front side56 has an optical coating 60. The back side 58 has an opaque feature 62.

The optical coating 60 reflects at least 90% of the optical power of thelight beam 26 along the main optical path 28. The X-mirror 42 transmitsno more than 10% of the optical power of the light beam 26 along thesecondary optical path 46. In a more particular embodiment, the X-mirror42 transmits between 0.1% and 4% of the optical power of the light beam26 along the secondary optical path 46. In a yet more particularembodiment, the X-mirror 42 transmits between 0.2% and 2% of the opticalpower of the light beam 26 along the secondary optical path 46. In afurther embodiment, the X-mirror 42 transmits about 1% of the opticalpower of the light beam 26 along the secondary optical path 46.

In the illustrated embodiment the opaque feature 62 is a small dot ofreflective material that is centrally located relative to thetransmitted optical path 46. The result is that the X-sensor receives alowered intensity light beam with a shadow from the opaque feature 62.The location of the shadow with respect to the transmitted light beamalong path 46 is indicative of an alignment of the light beam relativeto the scanning system 30.

FIG. 5 is a schematic diagram illustrating a portion of the scanningsystem 30 including the Y-mirror 44 and the Y-sensor 52. The Y-mirror 44has a substrate 64 an optical coating 66. The optical coating reflectsat least 90% of the optical power of the light beam 26 along the mainoptical path 28. The Y-mirror transmits no more than 10% of the opticalpower of the light beam 26 along the secondary optical path 50. In amore particular embodiment, the Y-mirror 44 transmits between 0.1% and4% of the optical power of the light beam 26 along the secondary opticalpath 50. In a yet more particular embodiment, the Y-mirror 50 transmitsbetween 0.2% and 2% of the optical power of the light beam 26 along thesecondary optical path 50. In a further embodiment, the Y-mirror 50transmits about 1% of the optical power of the light beam 26 along thesecondary optical path 50.

From the Y-mirror 44, both the reflected and transmitted portions of thelight beam 26 converge. In the illustrated embodiment, the diameter ofthe beam at the Y-sensor is the same as the diameter at the build plane32. The converging lens 68 is included in order to shorten the physicalpath length between the Y-mirror and the Y-sensor 52. In a particularembodiment, the light beam has a focused minimum diameter at the buildplane 32 and the Y-sensor 52. An alternative embodiment does not includeconverging lens 68. Then the physical optical path length between theY-mirror and the build plane 32 is the same as the physical optical pathlength between the Y-mirror and the Y-sensor 52.

The Y-sensor 52 is a camera or other sensor that captures a profile ofthe transmitted portion of beam 26. Because the beam diameter at sensor52 is the same as at the build plane 32, the resultant profile is a goodindicator of how well focused the beam 26 is upon build plane 32. Thecontroller 10 is configured to analyze the profile and to determine adegree of the focus of laser module 22 upon the build plane 32.

FIG. 6 is a schematic diagram of a portion of the three-dimensionalprinting system 2 for illustrating a last portion of the main opticalpath 28 from the scan module 30 to the build plane 32. During themanufacture of the three-dimensional article 14, fumes are generated.The scan module 30 is protected from the fumes by the base housing 33.The base housing 33 includes opening 34 for allowing the scanning beam26 to fully address the build plane 32. In some embodiments, the opening34 is a precision opening.

The opening 34 is closed by a transparent plate 72. The transparentplate 72 provides two functions. First, it protects the scan module 30from the fumes. Second, it provides calibration features for aligningthe optical path 28 to the build plane 32. The transparent plate 72includes a reflective feature 74. As the light beam 26 is scanned acrossbuild plane 32, it sometimes impinges on the reflective feature 74.Light is then reflected from reflective feature 74 up into light sensor76.

In an illustrative embodiment, the reflective feature 74 includes anarray of small dots or lines of reflective material with known locationscorresponding to the build plane 32. As the light beam 26 impinges uponthese small dots 74 the controller 10 receives signals indicative ofreflected light received by sensor 76. The controller 10 uses thisinformation to correlate operating of the scan module 30 with buildplane coordinates. In some embodiments, sensor 76 is an arrangement orarray of sensors 76 for capturing light reflected from different dots.Also, sensor 76 can represent different sensors optimized for differentkinds of measurement. One kind of measurement can be based upon presenceand/or trajectory of a light beam. Another kind of measurement can bebased upon a power level.

The small dots 74 are much smaller than a diameter of the converginglight beam 26 as it impinges upon the transparent plate 72. In oneembodiment, the light beam diameter (at impingement) is at least tentimes an axial dimension of a small dot 74. In other embodiments, thelight beam diameter at least 100 times or at least 1000 times thediameter of the small dot 74. The small dots 74 therefore have nosignificant effect on the light beam 26 insofar as properly solidifyingthe resin 12 at build plane 32.

In the illustrated embodiment, reflective feature 74 is disposed on anupper surface of the transparent plate 72. In an alternative embodiment,the reflective feature 74 can be disposed upon a lower surface of thetransparent plate 72. In some embodiments, the signal from sensor 76 canbe utilized to analyze a power output of the laser module 22. Having areflective feature 74 disposed upon the lower surface of the transparentplate 72 would allow a power level to include attenuation through thetransparent plate.

In the illustrated embodiment, an added reflective feature 75 is mountedbelow the transparent plate 72. This reflective feature 75 has an addedbenefit. As three-dimensional articles 14 are fabricated, a film ofresin can build up on the lower surface of the transparent plate 72.This will attenuate light from the scan module 30. During operation,sensor 76 can monitor an intensity of radiation reflected by reflectivefeature 75. Controller 10 can receive a signal from sensor 76 andanalyze that signal to determine a reduction in power level of beams 26reaching the build plane 32. Then the controller 10 can increase anenergy dosage in compensation and/or send a message to user interface 8instructing a user to clean the transparent plate 72.

Also included is an inclinometer 78 that provides a signal to thecontroller 10 that is indicative of the orientation of the scan module30 with respect to a gravitational reference. In one embodiment, thecontroller 10 is configured to provide instructions to a user forleveling portions of the three-dimensional printing system 2 in theevent that a certain threshold is determined. In another embodiment, thecontroller 10 is configured to compensate for an angular tilt.

FIGS. 7, 8, and 9 are flowcharts describing methods for calibrating thethree-dimensional printing system 2. The methods, described infra, areall performed by the controller 10. However, there may be manualprocesses associated with these methods.

FIG. 7 is a flowchart depicting an embodiment of a method 80 foraligning the light beam 26 to a main optical path 28. Method 80 shouldbe performed whenever a new laser is installed into the laser module 22.Method 80 can also be performed continuously during operation of thethree-dimensional printing system 2 in order to identify alignment driftso that corrective action can be taken.

According to 82, the laser module 22 is operated to generate and emit alight beam 26 along the main optical path 28. According to 84, the lightbeam 26 is received by a motorized mirror (e.g., X-mirror 42) having afront side optical coating 60 and a back side opaque feature 60. Lessthan 10% of the received radiative power is transmitted through theX-mirror 42. The X-sensor 48 receives at least some of the transmittedlight along optical path 46.

According to 86, the X-sensor 48 is operated to receive transmittedlight which includes a shadow from the opaque feature 62. Also,according to 86, the received light is analyzed to determine analignment of the beam 26 with respect to the main optical path 28.

According to 88, a determination is made as to whether the alignment ofbeam 26 to optical path 28 is out of tolerance. If the alignment is outof tolerance, further action is also taken according to 88. The furtheraction can include one or more of the following: (1) A message can besent to the user interface 8 that alerts the user regarding themisalignment. (2) Instructions can be sent to user interface 8 thatinstruct the user to manually utilize aligner 24 to align the lasermodule 22. (3) Instructions can be sent to operate an automatic aligner24 to automatically align the laser module 22.

FIG. 8 is a flowchart depicting an embodiment of a method 90 forcharacterizing the light beam 26 and taking further action as needed.According to step 92, the laser module 22 is operated to generate andemit a light beam 26 along the optical path 28. According to step 94,the light beam is received by a motorized mirror (e.g., Y-mirror 44)having a front side optical coating 66. Less than 10% of the receivedradiative power is transmitted through the Y-mirror. A sensor (e.g.,Y-sensor 52) receives the transmitted radiation.

According to 96, the Y-sensor 52 receives the transmitted radiation andanalyzes an intensity distribution. According to 98, a determination ismade as to whether the light beam 26 is out of focus at the Y-sensor 52.If the light beam 26 is out of focus, then further action is takeaccording to 98. The further action can include one or more of thefollowing: (1) A message can be sent to the user interface 8 that alertsthe user regarding the out of focus light beam 26. (2) Instructions canbe sent to the user interface 8 for manually adjusting the focus. (3) Aportion of the laser module 22 can be operated to auto-focus the lightbeam 26.

FIG. 9 is a flowchart depicting an embodiment of a method 100 foraligning a scanned light beam to a build plane using the apparatusdepicted in FIG. 6. According to 102, the laser module 22 is operated togenerate and emit a light beam 26 along the optical path 28 to thescanning system 30. According to 104, the scanning system 30 is operatedto scan the light beam 26 across the transparent window 72. According to106, the sensor 76 is operated to receive light from the reflectivefeature 74 and to generate a signal.

According to 108, the signal received from the sensor 76 is analyzed toalign the light beam 26 to the reflective feature 74. In doing so, thelight beam is aligned to the build plane 32.

In another embodiment, 108 includes determining a laser power from thesignal. In some embodiments measurements can be taken at different laserpower levels to provide a calibration at different power levels.

FIG. 10 is a method 110 of manufacturing a three-dimensional article 14using the three-dimensional printing system 2. According to 112, thecontroller positions active surface 17 at the build plane 32. As part of112, a layer of resin 12 is dispensed or otherwise made to cover theactive surface 17. According to 114, the laser module 22 and scan module30 are operated to selectively polymerize the resin 12 at build plane32.

According to 116, a signal is received from sensor 43. According to 118,a determination is made as to whether printing is complete. If so,printing is halted according to 120. If printing is not complete, thenthe signal is analyzed to see if the external vibration exceeded apredetermined threshold according to 122. If not, then the process loopsback to 112 for the formation of another layer. If the threshold hasbeen exceeded, then further action is taken according to 124.

The predetermined threshold for external vibration can be based on oneor more factors. One factor is a peak magnitude or amplitude of thevibration. Another factor is a duration of the vibration. Yet anotherfactor is an integration of vibrational energy over a certain timeperiod.

Further action according to 124 can include one or more actions taken bythe controller 10. A first action can be to halt the printing operationwhen the vibration is above a certain upper limit. Part of this firstaction can be to restart the printing operation when the vibration hasfallen to a certain lower limit. A second action can be to storeinformation concerning the vibration which can include one or more of atime stamp, a layer being formed, a position within thethree-dimensional article, and a vibration factor such as one used forcomparison with the threshold. A third action can be to send a warningto the user interface 8 that is indicative of a vibration being beyond athreshold or control limit.

In another implementation the further action vary depending upon morethan one predetermined threshold being exceeded. A higher threshold caninclude halting the printing. A lower threshold can include storinginformation concerning the vibration.

In yet other embodiments, step 116 can include receipt of signals fromother sensors. The further action 124 can include other adjustments suchas adjustments in timing for alignment.

The exact sequence of method 110 can vary. For example, step 116 canoccur concurrent with or between steps 112 and 114.

The specific embodiments and applications thereof described above arefor illustrative purposes only and do not preclude modifications andvariations encompassed by the scope of the following claims.

What is claimed:
 1. A three-dimensional printing system for solidifyinga build material at a build plane in a layer-by-layer manner comprising:a laser module for emitting a light beam along a main optical path fromthe laser module to the build plane; a scan module including a motorizedmirror and a sensor, the motorized mirror including a substrate with anoptical coating that reflects at least 90% of the light beam powerincident on the motorized mirror from the laser module to the buildplane as a primary light beam and transmits a remainder of the incidentlight beam power along a secondary optical path to the sensor; and acontroller configured to: operate the laser module to emit the incidentlight beam along the main optical path to the motorized mirror; operatethe scan module including the motorized mirror to scan the primary lightbeam over the build plane; analyze a signal from the sensor receivingthe remainder of the incident light beam power; and based upon theanalysis, estimate a calibration error for the laser module.
 2. Thethree-dimensional printing system of claim 1 wherein the optical coatingtransmits between 0.1 and 4.0 percent of the light beam power.
 3. Thethree-dimensional printing system of claim 1 wherein the optical coatingtransmits between 0.2 and 2.0 percent of the light beam power.
 4. Thethree-dimensional printing system of claim 1 wherein the optical coatingtransmits about one percent of the light beam.
 5. The three-dimensionalprinting system of claim 1 wherein the substrate includes a front sideand a back side, the back side including an opaque feature.
 6. Thethree-dimensional printing system of claim 5 wherein the opaque featureincludes a plurality of features.
 7. The three-dimensional printingsystem of claim 5 wherein the opaque feature is a reflective dot.
 8. Thethree-dimensional printing system of claim 5 wherein the transmittedlight includes a shadow formed by the opaque feature, the controlleranalyzes the transmitted light and compares a centroid of the shadowrelative to a centroid of the transmitted light to determine analignment error of the light beam relative to the mirror.
 9. Thethree-dimensional printing system of claim 1 wherein the motorizedmirror includes two motorized mirrors including an X-mirror and aY-mirror for scanning the light beam over the build plane, the mirrorsindividually have associated sensors including an X-sensor and aY-sensor to receive transmitted light from the X-mirror and the Y-mirrorrespectively and to provide separate signals to the controller.
 10. Thethree-dimensional printing system of claim 9 wherein the controlleranalyzes the signals from the sensors to determine at least twodifferent calibration parameters.
 11. The three-dimensional printingsystem of claim 1 wherein the controller is configured to analyze anintensity distribution of the transmitted light and to determine a focuserror for the laser module.
 12. The three-dimensional printing system ofclaim 1 wherein the light beam converges along the main optical pathbetween the mirror and the build plane and along a secondary opticalpath from the mirror to the sensor.
 13. The three-dimensional printingsystem of claim 12 wherein the beam diameter at the sensor issubstantially the same as the beam diameter at the build plane.
 14. Thethree-dimensional printing system of claim 12 further comprising aconverging optical element between the mirror and the sensor to shortena physical length of the secondary optical path.
 15. Thethree-dimensional printing system of claim 1 wherein the controller isconfigured to send an alert to a user interface when the calibrationerror exceeds a threshold.
 16. The three-dimensional printing system ofclaim 15 wherein the alert includes instructions for manually correctingthe calibration error.
 17. The three-dimensional printing system ofclaim 1 wherein the controller is configured to automatically operate anadjustment mechanism to correct the calibration error.
 18. Thethree-dimensional printing system of claim 1, wherein the secondaryoptical path leads directly to the sensor.
 19. The three-dimensionalprinting system of claim 1, wherein the secondary optical path isoutside the build plane.